As fixed type constant velocity universal joints, there have been publicly known joints of, for example, a so-called six-ball Rzeppa type (BJ) and a six-ball undercut-free type (UJ), and an eight-ball Rzeppa type (EBJ) and an eight-ball undercut-free type (EUJ). Those joints are used as appropriate in accordance with usage, required characteristics, and the like. Further, there have also been proposed various fixed type constant velocity universal joints of a so-called track groove crossing type (for example, Patent Literature 1).
Referring to FIG. 13A and FIG. 13B, description is given of a fixed type constant velocity universal joint of a track groove crossing type disclosed in Patent Literature 1. FIG. 13A is a vertical sectional view of a state in which the fixed type constant velocity universal joint disclosed in Patent Literature 1 forms an operating angle of 0°, and FIG. 13B is a vertical sectional view of a state in which the fixed type constant velocity universal joint forms an operating angle (maximum operating angle θmax: 47 °). The constant velocity universal joint 121 includes an outer joint member 122, an inner joint member 123, balls 124, and a cage 125. In the constant velocity universal joint 121, a plurality of (for example, eight) arc-shaped track grooves 127 are formed in a spherical inner peripheral surface 126 of the outer joint member 122. The track grooves 127 are formed so that planes including ball raceway center lines x of the track grooves 127 are inclined with respect to a joint axial line n-n and the track grooves 127 are adjacent to each other in a peripheral direction with their inclination directions opposite to each other (detailed illustration of states of the inclination is omitted). Further, although detailed illustration is omitted, a plurality of (for example, eight) arc-shaped track grooves 129 are formed in a spherical outer peripheral surface 128 of the inner joint member 123. The track grooves 129 are formed so as to be mirror-image symmetrical with the paired track grooves 127 of the outer joint member 122 with respect to a plane P including a joint center O at the operating angle of 0°. That is, the inner joint member 123 is assembled to an inner periphery of the outer joint member 122 so that the paired track grooves 127 and 129 cross each other.
As illustrated in FIG. 13A, curvature centers of the arc-shaped track grooves 127 of the outer joint member 122 and the arc-shaped track grooves 129 of the inner joint member 123 are each positioned at the joint center O. Each ball 124 is interposed in a crossing portion between the track groove 127 of the outer joint member 122 and the track groove 129 of the inner joint member 123, which are paired with each other. The balls 124 are held in pocket portions 125a of the cage 125 arranged between the outer joint member 122 and the inner joint member 123. Curvature centers of a spherical outer peripheral surface 132 and a spherical inner peripheral surface 133 of the cage 125 are each positioned at the joint center O. In the constant velocity universal joint 121, the paired track grooves 127 and 129 cross each other, and the balls 124 are interposed in those crossing portions. Therefore, when the joint forms an operating angle, the balls 124 are always guided in a plane bisecting an angle formed between axial lines of the outer joint member 122 and the inner joint member 123. As a result, rotational torque is transmitted at a constant velocity between the two axes.
As described above, the track grooves 127 and 129 of the outer joint member 122 and the inner joint member 123 are adjacent to each other in the peripheral direction with their inclination directions opposite to each other. Therefore, when both the joint members 122 and 123 rotate relative to each other, forces in the opposite directions are applied from the balls 124 to the pocket portions 125a of the cage 125 that are adjacent to each other in the peripheral direction. Due to the forces in the opposite directions, the cage 125 is stabilized at the position of the joint center O. Thus, a contact force between the spherical outer peripheral surface 132 of the cage 125 and the spherical inner peripheral surface 126 of the outer joint member 122, and a contact force between the spherical inner peripheral surface 133 of the cage 125 and the spherical outer peripheral surface 128 of the inner joint member 123 are suppressed. Accordingly, torque loss and heat generation are suppressed. As a result, it is possible to attain a constant velocity universal joint that is excellent in torque transmission efficiency and durability.